The inventor of this application in U.S. patent published No. 20030187434 A1 discloses a vertebral fixation device comprising a fixation seat, a fixation block, and a fastening bolt. The fixation seat is provided with a receiving slot for receiving a vertebral fixation rod. The fixation block is engaged with the fixation seat such that the vertebral fixation rod is pressed by the fixation block, and that two retaining edges of the fixation block are retained in two retaining recesses of the fixation seat. The fastening bolt is engaged with a threaded through hole of said fixation block such that one end of the fastening bolt presses against the vertebral fixation rod. Although this design improves the conventional backbone-fixing devices, there are at least the following defects which call for further improvements: 1) the contact of the spherical head fastening screw and the fixation rod is restricted to a thin line; and 2) the fixation block has to be fabricated precisely to well fasten the fixation rod in the receiving slot of the fixation seats. For this reason, the fixation rod is susceptible to unintentional displacement in a spinal surgery in progress. Such a displacement of the back-retrieving rod often affects adversely the outcome of the spinal surgery. U.S. Pat. No. 6,565,565 B1 discloses a device for securing spinal rods, which is similar to that described in the US patent published No. 20030187434 A1 with the common drawbacks.